Electroacoustical apparatus



Sept 8, 1942- l.. J. ANDERSON 2,295,376

ELECTROACOUSTICAL APPARATUS i Filed' Dec. 28, 1959 Bnventor 1Q "faille, 50m

, 9 Z3 Mmm Patented sept. s, 194g ELECTBOACOUSTICAL APPARATUS Leslie Anderson, Westmont, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application December 2s, 1939, serial No. 311,371.vl

9 claims. (ci. 1791-139) ploying a ribbon or other conductor mounted forv movement in a magnetic eld and having a section responsive to the pressure component of a sound wave and another section responsive to the pressure gradient component of the sound wave, as shown, for example, in the Olson and Weinberger Reissue Patent No. 19,115, the requirements of fundamental resonant frequency dictate a certain minimum length of ribbon consistent with.proper mechanical design. This minimum usually lies in the orderof one inch. When the acoustic line or pipe is coupled to the pressure responsive section of the ribbon, a considerable mismatch in sensitivity between the pressure responsive section and the pressure gradient, or velocity; responsive section may result. 'I'his is especially likely to be so if the acoustic line or pipe is of small physical size;

that is to say, of small diameter.

The conventional method of dealing with situation is to retain the mechanical length of the ribbon in the velocity section, but make a portion 'of the velocity section inactive by extending it beyond the magnetic field. This, of course, assumes that the pressure responsive section of the ribbon has the lower sensitivity, which is usually the case. Extending a portion of the ribbon beyond the air gap or magnetic eld is inefficient since it is equivalent to inserting a resistor inseries with the active portion of the velocity section.

The primary object of my present invention is to provide an improved uni-directional microphone `wherein the aforementioned diiiiculty is obviated.

More particularly, it is an object of my present invention to provide an improved uni-directional microphone of the type'set forth which Will have improved sensitivity.

In accordance with one form of my invention, I retain the entire length of ribbon in the velocity responsive section and reduce the impedance of the output circuit of said section. This may be done either by inserting a suitable resistor in shunt with the more sensitive ribbon section, that is, the velocity responsive section in the case assumed above, or by coupling the two ribbon sections to a suitable output transformer and ,tapping the common junction of the serially connected ribbon sections to a suitable tap on the this the impedance of the circuitincluding the velocity responsive section of the ribbon is reduced. Another Way of accomplishing this result is to couple each of the ribbon sections to the primary windings of two separate transformers with the ratio of the windings such as to give a reduced impedance in the circuit which includes the velocity responsive section.

In accordance with another form of my invention, I provide a magnetic structure in which .the

`pole pieces associated with the two ribbonv sections can be altered so as to provide a weaker magnetic field for one section than the other. By altering the flux densities in the twoair gaps, the sensitivities of the two ribbon sections can b made equal.y

The novel features that I consider 'characteristic of my invention vare set forth with particularity in the appended claims. itself,.however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description of several embodiments thereof when read in connection with the accompanying drawing, in which Figure 1 is a side elevation of a conventional form of uni-directional microphone,

Figure 2 is a wiring diagram showing the electrical equivalent of the conventional microphone,

Figure 3 is a wiring diagram according to one form of my present invention,

Figure 4 is a wiring diagram of 4another form of my present invention,

Figure 5 is a wiring diagram of still another form of my present invention,

vFigure 6 is a front elevation of a microphone -embodying still another form of my invention,

Figure 7 is a somewhat modified formv of the invention shown in Figure 6.

Referring more particularly jto the drawing,

wherein -similar reference characters designate corresponding parts throughout, there is shown in Figure 1, a conventional uni-directional microphone -of the type disclosed in the above identified reissue patent and comprising a conductor in the form of a crimped ribbon I mounted for movement in a suitable magnetic field between a pair of pole pieces 3. The ribbon I is clamped at its ends and also at its center to provide a pair of serially connected ribbon sections la and Ib. The ribbon section la is open to the atmosphere both at the front and at the back whereby it is responsive to the pressure gradient comprimary winding of .the transformer such that ponent of a plane sound wave. Behind the rib- The invention for cancellation.

bon section Ib is placed an acoustic resistance in the form of apipe l leaving only the front of the ribbon section Ib open to the atmosphere whereby the section I b isuresponsive to the pressure component of a plane sound wave. y

Ii the ribbon sections la and lb are of the ,same length, there results a, mismatch in sensitivity when the pipe 6 is added to the system, the section Ib becoming less sensitive. Because of the requirements of fundamental resonant frequency, the full length of each ribbon section must be retained. and to compensate for unequal sensitivity of the two ribbon sections, it has been customary to shorten the pole pieces 8 so that a portion ofthe velocity responsive ribbon section Vla will extend beyond the magnetic field. The eil'ect of this is equivalent to inserting a resistor in series with the ribbon section la, as shown in Figure 2 wherein the two ribbon sections are coupled to a suitable amplifier by a transformer 1, andthe velocity responsive ribbon section is divided into its active portion la' (that is, the portion thereof lying in the magnetic field) and its inactive portion la" (that is, the ribbon portion thereof extending beyond the magnetic' field).

Referring now, to Figure 2, if Rre-resistance of the pressure section Ib, Rz=resistance of the active portion la' of the a. velocity section la,

Rs=resistance of the inactive portion la" of the velocity section la,

1=voltage generated by the ribbon section la',

2=voltage generated by the ribbon section Ib,

p=voltage across the primary winding of the transformer 1,

o=voltage across the secondary winding of the transformer 1,

Z=impedance of the circuit of the secondary winding of the transformer 1, and l n=the ratio of tHe turns between the primary and secondary windings of the transformer 1, then where K is the power ratio.

Now, if it is assumed that the sensitivity of the velocity section la of the ribbon is twice that of the pressure section Ib if both of the velocity section portions |a' and la" were active, then,

Substituting v(4) and (5) in 3) (25:)s 4=2i (6) RVi-R1 2R! R! In Figure 3, I have shown'one way of increasing thel overall eiliciency of the microphone. In this case, I retain the whole length of the velocity ,section la within the magnetic ileld, so that its y whole length is active, and shunt a resistor 9 across thevelocity section la, thereby reducing the impedanceof the 'circuit of this section of the ribbon. Now, if R1=resistance of the pressure section Ib (as in the previous case),

R4=resistance of the velocity section la,

=resistance of the resistor i. 2=voltage generated by the pressure section Ib (as in the previous case), :voltage generated by the velocity section Ira. 4=voltage across the resistor 9, p=voltage across the primary winding of thc transformer 1, i :voltage'across the secondary winding of the transformer 1, Zp=impedance of the primary circuit of the transformer 1,

'Zo=impedance of the secondary circuit of the transformer 1, and

substituting (12) and (13) m (14). and simplifving Kleeman' If it is assumed that the velocity section 1a is ttgice as sensitive as the pressure section Ib,

2:2212 (16) and for cancellation R5 l mz (17) Substituting (16) and (17) in (15) Aand assumins that the'ribbon sections iaand ibare of the same length, so that Rr=R, then (18) R1 +2l Now,- dividing (18) by (6) the power gain will be 2l2'ezl 33; 2[,] 3

In other words, when the whole ribbon section la is made active and the resistor 9 is shunted acrom it, the power gain is 1.33 or about 1.25 db.

Figure 4 shows another lwayof improving the sensitivity of the system according to this invention while employing the full length of the velocity section la of the ribbon. Here, again, the impedance of the circuit of the velocity responsive section la is reduced relative to that of the circuit of the pressure responsive section la,

but in this case, I accomplish this result by emplaying a tapped primary winding for the transformer 'l and connect the common junction Ii of the serially connected ribbon sections la and lb to a tap I3 on the primary winding ofthe transformer such that this result will be effected. Let R1=resistance of the pressure section ib (as before), R4=resistance of the velocitysection ia (as before), z=voltage generated by the pressure section Ib (as before), a=voltage generated by the velocity section la (as before),

s=voltage across the portion 'la of the primary winding of the transformer due to 't=vo1tage across vthe portion la of the primary p winding of the transformer due to t=`voltage across the portion 'Ibof the primary winding of the transformer due to s, eis=voltage across the portion 'Ib of the primary winding of the transformer due to s, 1=tota1 voltage across the whole primary winding of the transformer due to s, "z=total voltage across the whole primary winding of the transformer due to ez,

`Zi=impedance looking into the portion la of the primary winding of the transformer, Z2=impedance looking into the portion 1b of the primary winding of the transformer, and X=the turn ratio between the yprimary winding portions 1a and 1b.

Assuming that the two ribbon sections la and Ib are of equal length and therefore of equal resistance, then Substituting (|21) and (24) in (25) and simplifying For cancellation, t must equal 'i, whence, from (26) and (32) v Then, from the assumption made above leading Since p=1+'1 and` 7='1 from the assumption made above leading to (33), then Hence, the gain in eiliciency oiered by this modidcation of my invention over the conventional system of Fig. 2, dividing (may 43) by c)= LSR or a gain of approximately 2 db.

In Fig. 5, I have shown a modification o f my invention which is similar to that shown in Fig.

4 but employing two separate transformers I! and I1, respectively, for the ribbon sections la and ib. The secondary windings of the transformers I5 and II may be connected either in series or in parallel depending upon the conditions of use, although the series connection, as

y shown, is preferred since, under conditions of a loaded output, the series connection gives better results.

If, as in Fig. 4, the resistances of the ribbon 4 sections la and 1b are equal andhave a value R and :voltage generated by the pressure section Ib, :voltage generated by the velocity section Ia, s :voltage across the secondary winding of the transformer I5,

s :voltage across the secondary winding of the transformer I1,

o :total output voltage across the secondary windings of both transformers I5 and I1,

Zs=the impedance looking into the seconda winding of the transformer I5,

Zn-the impedance looking into the secondary winding of the transformer I1,

Zo=total output impedance as viewed from both secondary windings of the two transformers,

ms=ratio of turns between the two windings of the transformer I5, and

n11=ratio of turns between the two windings of the transformer I1,

then

Zc=Za+Ze (44) Zs=('n15)2R (45) and Zsf=(1l17)2R v (46) Substituting (44) and (45) in (46),

Zo=(ms)2R-I(nrz)2R- (47) Now For a unidirectional microphone, the condiin the resulting equation, the gain in efficiency of the system of Fig.- 5 over the conventional system of F'ig. 2 is Here, again, therefore, there is a gain of about 2 db. over the conventional system.`

In Fig 6. I have shown a still further modifica- 'tlon of my invention wherein the field structure associated with the two ribbon sections can be :altered so as to provide a weaker magnetic eld for one section than for the other. In this modication, the ribbon sections Ia and Ilrm'ay be separated, if desired, and their ends clamped between suitable blocks 2I in a well known manner. One or more magnets 23 associated with the pole pieces 5 supply the necessary flux in the air gap. When properly designed, the ux density in the pole air gap will be uniform. Since, as was assumed in connection with the previous described modications, the velocity section Ia is more sensitive than the pressure section Ib, in

, order to obtain theksame output from. each ribclaims. i

bon section, it is necessary to make the flux density in that portion of the air gap where the velocity section Ia is mounted weaker thanin that portion thereof where the pressure section Ib is mounted. To accomplish this, I split the pole pieces 3 land insert brass or other suitable non-magnetic blocks 25 therebetween to provide a reluctance. In thisway, there will be less flux flowing through the air gap of the velocity section Ia than through the air gap of the pressure section Ib. The length of the two ribbons or ribbon sections Ia and Ib are kept the same as before. If desired, the pole. pieces 3 may be formed somewhat L-shaped, as shown in Fig. 7, and the adjacent split sections thereof connected by means of a screw 21 or the like of brass so that the sections of the pole pieces may be made adjustable relative to each other andA the reluctance thereby varied.

From the foregoing description, it will be apparent that I have provided an improved unidirectional microphone which has greater sensitivity than conventional microphones of this type. Although I have shown and described several modifications of my invention, it will be apparent to those skilled in the art that many other modifications, as well as changes in those herein described, are possible. I therefore. wish it to be understood that I do not intend to be .limited except insofar as is made necessary by the prior art and by the spirit of the appended I claim as my invention: 1. In a sound translating system employing a unidirectional microphone having a conductor movable in a magnetic field, said conductor having two sections of unequal sensitivities, .the method of increasing the overall sensitivity of said system which comprises reducing the eiective impedance of the niore sensitive of said conductor sections.

2. In a sound translating system employing a unidirectional microphone having a conductor movable in a magnetic field, said conductor having two sections of unequal sensitivities, the method of increasing the overall sensitivity of said system -which comprises shunting a resistkance across 'the more sensitive section whereby to reduce the overallimpedance of the circuit of said last named section.

3. In a sound translating system employing a unidirectional microphone having a conductor movable in a magnetic field, said conductor having two sections of unequal sensitivities; the method of increasing the overall sensitivity of said system which comprises coupling said sections to the primary winding of a transformer and connecting the common ends of said sections to a tap on said primary winding such that the circuit of the more sensitive one of said sections has a lower impedance than the circuit including the other of said sections.

4. In a soundtranslating system employing a A movable inal magnetic eld, said conductor hav= ing two sections ofunequal sensitivities, the method of increasing the overall sensitivity of said system which comprises coupling each of said sections to the primary winding of a pair of transformers and so choosing the ratio of the turns of the winding in circuit with the more sensitive one of said sections to the turns of the winding in circuit with the less sensitive one of said windings that the impedance of said first named circuit will be less than the impedance of said second named circuit.

5. In a sound translating system, the combination of a uni-directional microphone `having a sound wave pressure responsive element and a sound wave pressure gradient responsive element, said elements being serially connected, and an output transformer therefor, said elements being connected to the primary winding of said transformer, and the common junction of said elements being connected to a tap on said transformer such that the impedance of the circuit of saidpressure gradient responsive section is lower than the impedance of the circuit of said other element.

6. In a sound translating system, the combination o'f a uni-directional microphone having a sound wave pressurevresponsive element and a sound wave pressure gradient responsive element, said elements being serially connected, a pair of `output transformers, said rst named element being connected to the primary winding of one of said transformers, and said second named element being connected to the primary winding of the other of said transformers, and the secondary windings oi' said transformers being connected together, the ratio oi the turns of said first named primary winding to the turns of said second named primary winding being such that the impedance of the circuit o f said second named v 5 element is lower the impedance of the circuit of said nrst-named element.

'1. In a sound translating system employing a uni-directional microphone having a pair of conductors movable in magnetic fields, one of said conductors being responsive to the velocity component and the other to the pressure component of a sound wave and said rst named conductor \being more sensitive than said second named conductor, the method of increasing the overall.

sensitivity of said system which comprises reducing the eiective impedance ot said rst `named conductor.

8. In a microphone system, the combination of y means providing a magnetic field, a -pair of vibratile conductor sections disposed in said mag netic field and arranged to be actuated by sound IWaves, one of said sections being capable of producing a greater voltageothan the other of said sections in response to sound waves, and means connected in shunt relation with said first named .section for lowering the impedance thereof an v amount to substantially equalize the voltage outputs from said sections.

9. In a microphone system, the combination of means providing a magnetic field. a. pair of vibratile conductor sections disposed in said magnetic eld and arranged to be actuated by sound waves, one of said sections being capable of producing a greater voltagethan the other of said sections in response to sound waves, and a transformer having two primary windings connected one to eachof said sections, respectively, the ratios of transformation of said windings dinering to such an extent and in such direction that the effective impedance of said rst named sec-v tion, as viewed from the secondary winding of said transformer, is lowered and the overall sensi- 1 tivity oi said system is increased.

LESLIE J. ANDERSON. e 

